The government is hell-bent on finishing the job started by Margaret Thatcher in 1979. When Thatcher came to power, on the back of the much-exaggerated "winter of discontent," Britain had 13.5 million trade union members. Today, the number hovers over 6 million.

In 1979, union membership spread across private and public sectors. There were the muscular battalions of the manufacturing and production industries. Two hundred thousand miners provided Britain's main source of energy, coal. Manufacturing, though in decline, still had a strong union base – engineering, tailoring, textiles, printing. It took Thatcher a decade to destroy those battalions, through a combination of legislation, tactics and decline. In 1979, people on strike could claim benefits to help them survive. The benefit rules were changed, putting a stop to that.

Strikers could claim back income tax because of reduced earnings while on strike. That was stopped – no tax could be reclaimed until strike action ended. Legislation imposed new procedures on how unions were run, how and when strikes could take place, making the organising of strikes more complex and difficult. Thatcher took on the miners, and beat them, using a nationally organised police force, blatant violence and tactics that reduced mining communities to hunger.

With the miners beaten, powerful industries were privatised and broken up. What had been public sector became private sector – telecom and the railways, electricity and gas production, the docks. Big public sector operations became fragmented. More legislation shackled working people's ability to organise in unions. Employers no longer had to recognise a union even when the majority in a workplace voted to be represented (the Tories then, as now, were choosey about who was entitled to the benefits of democracy). Coupled with the legislative and confrontational warfare against the unions was the decline in manufacturing, which decimated many of Britain's traditional industries. My own city, Leeds, saw the disappearance of tens of thousands of jobs in engineering, tailoring and textiles.

Lost jobs meant lost union membership – and Thatcher's era saw unemployment climb to more than four million, many from union-organised industries. The result was that dozens of unions could not survive financially. They were gobbled up by bigger unions, merged out of existence.

Throughout this process Thatcher could rely on a supportive media which – just like today – peddled myths about "union barons" and "greedy union members" who "held the country to ransom". Mergers have led to the emergence of two giant unions. Unite, which straddles private and public sectors, has a listed membership of 1.4m, though a leaked report from its retiring general secretary allegedly says membership is in decline. Unison covers 1.3m jobs in the public sector, including the NHS and local authorities – an area of growing trade union membership, presumably as a result of public service workers' recognition of the threat they face. The general union GMB, with 600,000 members straddles both sectors. The Public and Commercial Services union PCS represents 300,000 civil service staff. Also in the public sector are the teaching unions, with the two biggest representing well over half a million people. Usdaw represents 400,000 workers in the shop, retail and distribution trades.

These are today's big battalions, among the 55 unions left in Britain. Today's threat to the biggest remaining area of trade union organisation in Britain is greater than that posed by Thatcher in the 1980s, including privatisation. Britain's current anti-union legislation makes it one of the most repressive countries in the European Union for working people to organise. To its shame Labour did not lift this repressive legislation.

Despite this situation, what is left of the trade union movement in Britain is still potentially a force to defend not only the working conditions, wages, jobs and pensions of its members, but also the public services on which so many millions of people rely. It is to be hoped the unions use their strength effectively, and also win the vital battle for public support.

The anti-public sector, anti-union campaign is being supported once again by the Tory-dominated sections of the media. "Gold-plated pensions" is one battle-cry. Tell that to a public sector pensioner who worked 40 years to get £70 a week in retirement. If the public sector unions lose this battle, then wave goodbye to the public services on which you rely, and which are already beginning to disappear. Think about it, then join a picket line on 30 June.

• This article was amended on 27 June, 2011. The sentence "Usdaw represents 386,000 workers in the shop, retail and distribution trades" in the sixth paragraph was changed to read "Usdaw represents 400,000 workers in the shop, retail and distribution trades" at the request of Usdaw, to reflect more up to date figures.